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Message-Id: <AC2A955C-3E0C-4A4D-879A-38B715A86149@gmail.com>
Date: Wed, 20 Nov 2019 14:56:56 +0300
From: Artem Blagodarenko <artem.blagodarenko@...il.com>
To: Alex Zhuravlev <azhuravlev@...mcloud.com>
Cc: "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [RFC] improve malloc for large filesystems
Hello Alex,
Thank you for this important topic. I am going to test your patch and give feedback. But this requires some time.
Now I want to share my thougths about this topic.
Here are slides from LAD2019 “Ldiskfs block allocator and aged file system" https://www.eofs.eu/_media/events/lad19/14_artem_blagodarenko-ldiskfs_block_allocator.pdf
There is also patch https://lists.openwall.net/linux-ext4/2019/03/11/5 that was sent here, but it require to be improved.
Best regards,
Artem Blagodarenko
> On 20 Nov 2019, at 13:35, Alex Zhuravlev <azhuravlev@...mcloud.com> wrote:
>
> Hi,
>
> We’ve seen few reports where a huge fragmented filesystem spends a lot of time looking for a good chunks of free space.
> Two issues have been identified so far:
> 1) mballoc tries too hard to find the best chunk which is counterproductive - it makes sense to limit this process
> 2) during scanning the bitmaps are loaded one by one, synchronously - it makes sense to prefetch few groups at once
> Here is a patch for comments, not really tested at scale, but it’d be great to see the comments.
>
> Thanks in advance, Alex
>
> diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
> index 0b202e00d93f..76547601384b 100644
> --- a/fs/ext4/balloc.c
> +++ b/fs/ext4/balloc.c
> @@ -404,7 +404,8 @@ static int ext4_validate_block_bitmap(struct super_block *sb,
> * Return buffer_head on success or NULL in case of failure.
> */
> struct buffer_head *
> -ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group)
> +ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group,
> + int ignore_locked)
> {
> struct ext4_group_desc *desc;
> struct ext4_sb_info *sbi = EXT4_SB(sb);
> @@ -435,6 +436,13 @@ ext4_read_block_bitmap_nowait(struct super_block *sb, ext4_group_t block_group)
> if (bitmap_uptodate(bh))
> goto verify;
>
> + if (ignore_locked && buffer_locked(bh)) {
> + /* buffer under IO already, do not wait
> + * if called for prefetching */
> + err = 0;
> + goto out;
> + }
> +
> lock_buffer(bh);
> if (bitmap_uptodate(bh)) {
> unlock_buffer(bh);
> @@ -524,7 +532,7 @@ ext4_read_block_bitmap(struct super_block *sb, ext4_group_t block_group)
> struct buffer_head *bh;
> int err;
>
> - bh = ext4_read_block_bitmap_nowait(sb, block_group);
> + bh = ext4_read_block_bitmap_nowait(sb, block_group, 1);
> if (IS_ERR(bh))
> return bh;
> err = ext4_wait_block_bitmap(sb, block_group, bh);
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 03db3e71676c..2320d7e2f8d6 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1480,6 +1480,9 @@ struct ext4_sb_info {
> /* where last allocation was done - for stream allocation */
> unsigned long s_mb_last_group;
> unsigned long s_mb_last_start;
> + unsigned int s_mb_toscan0;
> + unsigned int s_mb_toscan1;
> + unsigned int s_mb_prefetch;
>
> /* stats for buddy allocator */
> atomic_t s_bal_reqs; /* number of reqs with len > 1 */
> @@ -2333,7 +2336,8 @@ extern struct ext4_group_desc * ext4_get_group_desc(struct super_block * sb,
> extern int ext4_should_retry_alloc(struct super_block *sb, int *retries);
>
> extern struct buffer_head *ext4_read_block_bitmap_nowait(struct super_block *sb,
> - ext4_group_t block_group);
> + ext4_group_t block_group,
> + int ignore_locked);
> extern int ext4_wait_block_bitmap(struct super_block *sb,
> ext4_group_t block_group,
> struct buffer_head *bh);
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index a3e2767bdf2f..eac4ee225527 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -861,7 +861,7 @@ static int ext4_mb_init_cache(struct page *page, char *incore, gfp_t gfp)
> bh[i] = NULL;
> continue;
> }
> - bh[i] = ext4_read_block_bitmap_nowait(sb, group);
> + bh[i] = ext4_read_block_bitmap_nowait(sb, group, 0);
> if (IS_ERR(bh[i])) {
> err = PTR_ERR(bh[i]);
> bh[i] = NULL;
> @@ -2095,10 +2095,52 @@ static int ext4_mb_good_group(struct ext4_allocation_context *ac,
> return 0;
> }
>
> +/*
> + * each allocation context (i.e. a thread doing allocation) has own
> + * sliding prefetch window of @s_mb_prefetch size which starts at the
> + * very first goal and moves ahead of scaning.
> + * a side effect is that subsequent allocations will likely find
> + * the bitmaps in cache or at least in-flight.
> + */
> +static void
> +ext4_mb_prefetch(struct ext4_allocation_context *ac,
> + ext4_group_t start)
> +{
> + ext4_group_t ngroups = ext4_get_groups_count(ac->ac_sb);
> + struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
> + struct ext4_group_info *grp;
> + ext4_group_t group = start;
> + struct buffer_head *bh;
> + int nr;
> +
> + /* batch prefetching to get few READs in flight */
> + if (group + (sbi->s_mb_prefetch >> 1) < ac->ac_prefetch)
> + return;
> +
> + nr = sbi->s_mb_prefetch;
> + while (nr > 0) {
> + if (++group >= ngroups)
> + group = 0;
> + if (unlikely(group == start))
> + break;
> + grp = ext4_get_group_info(ac->ac_sb, group);
> + /* ignore empty groups - those will be skipped
> + * during the scanning as well */
> + if (grp->bb_free == 0)
> + continue;
> + nr--;
> + if (!EXT4_MB_GRP_NEED_INIT(grp))
> + continue;
> + bh = ext4_read_block_bitmap_nowait(ac->ac_sb, group, 1);
> + brelse(bh);
> + }
> + ac->ac_prefetch = group;
> +}
> +
> static noinline_for_stack int
> ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> {
> - ext4_group_t ngroups, group, i;
> + ext4_group_t ngroups, toscan, group, i;
> int cr;
> int err = 0, first_err = 0;
> struct ext4_sb_info *sbi;
> @@ -2160,6 +2202,9 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> * cr == 0 try to get exact allocation,
> * cr == 3 try to get anything
> */
> +
> + ac->ac_prefetch = ac->ac_g_ex.fe_group;
> +
> repeat:
> for (; cr < 4 && ac->ac_status == AC_STATUS_CONTINUE; cr++) {
> ac->ac_criteria = cr;
> @@ -2169,7 +2214,15 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> */
> group = ac->ac_g_ex.fe_group;
>
> - for (i = 0; i < ngroups; group++, i++) {
> + /* limit number of groups to scan at the first two rounds
> + * when we hope to find something really good */
> + toscan = ngroups;
> + if (cr == 0)
> + toscan = sbi->s_mb_toscan0;
> + else if (cr == 1)
> + toscan = sbi->s_mb_toscan1;
> +
> + for (i = 0; i < toscan; group++, i++) {
> int ret = 0;
> cond_resched();
> /*
> @@ -2179,6 +2232,8 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> if (group >= ngroups)
> group = 0;
>
> + ext4_mb_prefetch(ac, group);
> +
> /* This now checks without needing the buddy page */
> ret = ext4_mb_good_group(ac, group, cr);
> if (ret <= 0) {
> @@ -2872,6 +2927,9 @@ void ext4_process_freed_data(struct super_block *sb, tid_t commit_tid)
> bio_put(discard_bio);
> }
> }
> + sbi->s_mb_toscan0 = 1024;
> + sbi->s_mb_toscan1 = 4096;
> + sbi->s_mb_prefetch = 32;
>
> list_for_each_entry_safe(entry, tmp, &freed_data_list, efd_list)
> ext4_free_data_in_buddy(sb, entry);
> diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
> index 88c98f17e3d9..9ba5c75e6490 100644
> --- a/fs/ext4/mballoc.h
> +++ b/fs/ext4/mballoc.h
> @@ -175,6 +175,7 @@ struct ext4_allocation_context {
> struct page *ac_buddy_page;
> struct ext4_prealloc_space *ac_pa;
> struct ext4_locality_group *ac_lg;
> + ext4_group_t ac_prefetch;
> };
>
> #define AC_STATUS_CONTINUE 1
> diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
> index eb1efad0e20a..4476d828439b 100644
> --- a/fs/ext4/sysfs.c
> +++ b/fs/ext4/sysfs.c
> @@ -198,6 +198,9 @@ EXT4_RO_ATTR_ES_UI(errors_count, s_error_count);
> EXT4_ATTR(first_error_time, 0444, first_error_time);
> EXT4_ATTR(last_error_time, 0444, last_error_time);
> EXT4_ATTR(journal_task, 0444, journal_task);
> +EXT4_RW_ATTR_SBI_UI(mb_toscan0, s_mb_toscan0);
> +EXT4_RW_ATTR_SBI_UI(mb_toscan1, s_mb_toscan1);
> +EXT4_RW_ATTR_SBI_UI(mb_prefetch, s_mb_prefetch);
>
> static unsigned int old_bump_val = 128;
> EXT4_ATTR_PTR(max_writeback_mb_bump, 0444, pointer_ui, &old_bump_val);
> @@ -228,6 +231,9 @@ static struct attribute *ext4_attrs[] = {
> ATTR_LIST(first_error_time),
> ATTR_LIST(last_error_time),
> ATTR_LIST(journal_task),
> + ATTR_LIST(mb_toscan0),
> + ATTR_LIST(mb_toscan1),
> + ATTR_LIST(mb_prefetch),
> NULL,
> };
> ATTRIBUTE_GROUPS(ext4);
>
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